Ronald Knox (1888 – 1957) was a Golden Age writer of mystery stories (1920s and 30s) and a member of the Detection Club,  a group of mystery writers which also included Agatha Christie, Dorothy Sayers, and John Dickson Carr, among others.

Knox was monsignor of the Catholic Church and also wrote many religious works, but he may be best known for his Ten Commandments or Rules for Detective Fiction. The purpose of the commandments was to distinguish the whodunit style of mystery (where the clues are fairly given to the reader who will then have a chance of solving the crime with the detective) from other stories involving crime and detectives, such as the hard-boiled novels where the detective goes around punching other people out (or getting punched out himself) when he’s not making love to the beautiful client who usually turns out to be the culprit.

Here are Knox’s Commandments, and please note that they were written in a different time when different sensibilities were the rule. Watch out especially for number 5.

Knox’s Commandments:

1. The criminal must be someone mentioned in the early part of the story, but must not be anyone whose thoughts the reader has been allowed to follow.
2. All supernatural or preternatural agencies are ruled out as a matter of course.
3. Not more than one secret room or passage is allowable.
4. No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end.
5. No Chinaman must figure in the story. 
6. No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right.
7. The detective must not himself commit the crime.
8. The detective must not light on any clues which are not instantly produced for the inspection of the reader.
9. The stupid friend of the detective, the Watson, must not conceal any thoughts which pass through his mind; his intelligence must be slightly, but very slightly, below that of the average reader.
10. Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.

As I said, it was a different time.

And yes, most of the best writers of whodunits like Christie and Carr broke those rules with wild abandon in their best novels all the time. But they still managed to play scrupulously fair with the reader in laying out all the clues; they simply did so in such a devilishly clever way that most readers were always looking in all the wrong places.